1. Field of the Invention
The present invention relates to an electromagnetic coupling apparatus such as brake or clutch and a drive force distributing apparatus for a vehicle using the electromagnetic coupling apparatus.
2. Description of the Related Art
A differential is located in a power train of a vehicle to maintain torque distribution between right and left wheels of the vehicle such that torque is equally divided between the right and left wheels and to rotate the outside wheel faster than the inside wheel in cornering, thereby reliably obtaining smooth cornering. While the primary role of the differential is to obtain smooth cornering as mentioned above, there is a case that one of the right and left wheels may be caught to slip in a muddy place during rough-road running. In this case, the resistance from the road to the wheel caught to slip in the muddy place is small, so that torque is almost transmitted to this slipping wheel and hardly transmitted to the other wheel. As a result, the drive force for driving the wheels becomes lacking to cause a problem that the slipping wheel cannot escape from the muddy place. This problem is a defect inherent to a general differential.
Known is a special type of differential having a differential motion limiting mechanism capable of compensating for the above inherent defect of a general differential. This type of differential is referred to as a limited slip differential (LSD). A planetary gear type differential is generally known in the art. For example, such a planetary gear type differential gear assembly having a limited slip differential mechanism composed of an electromagnetic clutch and a multiplate clutch is disclosed in Japanese Patent Laid-open No. Hei 6-33997. In this differential gear assembly, an attraction force between a solenoid and an armature forming the electromagnetic clutch is applied to the multiplate clutch to press it and selectively control an engaging force generated in the multiplate clutch.
A connecting member consisting of a plurality of bars is located between a pressure plate of the multiplate clutch and the armature. That is, one end of each bar of the connecting member is fixed to the pressure plate of the multiplate clutch, and the other end comes into abutment against an inner circumferential portion of the armature when the solenoid is operated. In the conventional differential gear assembly mentioned above, the plural bars fixed to the pressure plate extend in a direction substantially perpendicular to the pressure plate. Accordingly, in the case that any of these bars are inclined to the pressure plate, there is a problem that a pressing force of the armature attracted by the solenoid to press the pressure plate of the multiplate clutch may not be uniformly transmitted to the pressure plate.
Further, in the conventional differential gear assembly described in the above publication, the electromagnetic clutch controls the engaging force of the multiplate clutch, so that the plural bars as pressure members are located so as to correspond to the inner circumferential portion of the armature. However, in a multiplate brake structure having a plurality of brake plates and a plurality of brake discs, these brake plates and brake discs are generally located so as to correspond to an outer circumferential portion of the armature from the viewpoint of the structure. Accordingly, it is difficult that the conventional structure described in the above publication such that the multiplate clutch is operatively connected to the armature at its inner circumferential portion is applied to the multiplate brake structure without any changes.
In the differential gear assembly described in the above publication, the opposed portion between the armature and a core having a solenoid coil is formed by flat opposed surfaces. This structure has a problem that high accuracy is required for control of an air gap between the core and the armature, because the relation between current and attraction force is sensitive to a change in this air gap.
Japanese Patent Laid-open No. Hei 11-260632 discloses a technique such that the opposed surfaces of a linear solenoid core and an armature are inclined with respect to their radial directions to increase the opposed area between the core and the armature, thereby improving the responsivity. Japanese Utility Model Laid-open No. Hei 6-26213 discloses an electromagnet configured so that one of the opposed surfaces of a core and an armature is formed with a V-groove and the other is formed with a wedge to be fitted with the V-groove. In each of these techniques described in the above publications, the opposed surfaces of the core and the armature are inclined with respect to their radial directions, so as to increase the opposed area between the core and the armature, thereby improving the responsivity. However, no attention has been paid to such an object of the present invention that the relation between current and attraction force is to be insensitive to a change in the air gap.
It is therefore an object of the present invention to provide an electromagnetic coupling apparatus which can make the relation between current and attraction force insensitive to a change in the air gap between the core and the armature.
In accordance with an aspect of the present invention there is provided an electromagnetic coupling apparatus interposed between a fixed housing and a rotating member at least partially accommodated in the fixed housing. This electromagnetic brake includes a multiplate brake mechanism having a plurality of brake plates mounted on the fixed housing and a plurality of brake discs mounted on the rotating member so as to be arranged in alternate relationship with the brake plates; a ringlike core member fixed in the fixed housing, said core member having an annular groove and a first outer diameter; an annular exciting coil accommodated in the annular groove of the core member; and a ring-like armature member arranged in opposed relationship with the annular groove of the core member, said armature member having a second outer diameter larger than the first outer diameter.
The electromagnetic coupling apparatus further includes a cylindrical pressure member provided so as to surround the outer circumferential surface of the core member and be movable in a direction of pressing the multiplate brake mechanism as being guided by the core member, said pressure member having a first end fixed to an outer circumferential portion of the armature member and a second end engaged with the multiplate brake mechanism. The term xe2x80x9celectromagnetic coupling apparatusxe2x80x9d may include an electromagnetic brake and an electromagnetic clutch.
The ringlike core member has a first radially outside tapered end surface formed radially outside of the annular groove and inclined at a first angle with respect to the axis of the core member, and a first radially inside tapered end surface formed radially inside of the annular groove and inclined at a second angle with respect to the axis of the core member. The ringlike armature member has a second radially outside tapered end surface complementary to the first radially outside tapered end surface, a second radially inside tapered end surface complementary to the first radially inside tapered end surface, and an intermediate end surface opposed to the annular exciting coil.
If the air gap between the ringlike core member and the ringlike armature member changes in the electromagnetic brake having the multiplate brake mechanism, the attraction force of the ringlike core member changes to result in a change in the pressing force of the cylindrical pressure member to the multiplate brake mechanism. Therefore, high accuracy is required for control of the air gap. This air gap changes according to manufacturing variations or aged deterioration (wearing) of the plural brake plates and the plural brake discs of the multiplate brake mechanism.
For example, in the case that the opposed portion between the ringlike core member and the ringlike armature member is flat so as to extend at right angles to the axis, there is a possibility that when the air gap formed by passing a large current through the annular exciting coil to maximize the attraction force of the core member is gradually decreased by the aged deterioration, the pressing force of the cylindrical pressure member may gradually become stronger than an original pressing force. This is due to the fact that the amount of wear of the brake plates and the brake discs of the multiplate brake mechanism due to the aged deterioration appears in the amount of movement (stroke) of the cylindrical pressure member and that the stroke of the cylindrical pressure member and the air gap are in a one-to-one correspondence because the opposed portion between the core member and the armature member is flat so as to extend at right angles to the axis.
According to the invention described above, the opposed portion between the ringlike core member and the ringlike armature member is formed by the tapered surfaces each having a predetermined tapered angle. Accordingly, even when the stroke of the cylindrical pressure member is changed by the aged deterioration of the multiplate brake mechanism, the air gap decreases by an amount corresponding to the tapered angle, so that the relation between the stroke of the cylindrical pressure member and the air gap becomes a one-to-one or less correspondence. Accordingly, a change in the air gap with a change in the stroke of the cylindrical pressure member due to wearing in the multiplate brake mechanism can be suppressed. As a result, the control of the attraction force of the ringlike core member, or the control of the braking force in the multiplate brake mechanism can be accurately performed without much influence by the aged deterioration.
In accordance with another aspect of the present invention, there is provided an electromagnetic coupling apparatus wherein the second angle is set larger than the first angle. At the outer circumferential portion of the ringlike armature member where the cylindrical pressure member is fixed, the attraction force of the ring-like core member is applied to the armature member. At this time, the air gap between the radially outside tapered end surface of the armature member and the radially outside tapered end surface of the core member can be ensured even when the attraction force becomes maximum, because the cylindrical pressure member is connected to the armature member at its outer circumference. However, there is a possibility that the air gap between the radially inside tapered end surface of the armature member and the radially inside tapered end surface of the core member may become zero because of tilt, deflection, etc. of the armature member, causing the contact of the core member and the armature member at their inner circumferential portions.
According to the another aspect of the present invention, the second angle is set larger than the first angle. With this configuration, the air gap between the radially inside tapered end surfaces of the core member and the armature member can be preliminarily set larger than the air gap between the radially outside tapered end surfaces of the core member and the armature member, thereby improving the margin for contact between the core member and the armature member.
In accordance with a further aspect of the present invention, there is provided an electromagnetic coupling apparatus wherein the second radially outside tapered end surface and the second radially inside tapered end surface of the ringlike armature member form a conical projection. In other words, the second radially outside tapered end surface and the second radially inside tapered end surface of the ringlike armature member are formed so as to reduce the volume of the armature member. Accordingly, the moment of tilt of the armature member can be reduced to improve the margin for contact between the core member and the armature member.
In accordance with a still further aspect of the present invention, there is provided an electromagnetic coupling apparatus interposed between a fixed housing and a rotating member at least partially accommodated in the fixed housing. This electromagnetic coupling apparatus includes a multiplate brake mechanism having a plurality of brake plates mounted on the fixed housing and a plurality of brake discs mounted on the rotating member so as to be arranged in alternate relationship with the brake plates; a first ringlike core member fixed in the fixed housing, said first ringlike core member having an annular groove, a first outer diameter, and a first inclined end surface inclined at a first angle with respect to the axis of the first ringlike core member; and a second ringlike core member fixed to the first ringlike core member, said second ringlike core member having a second inclined end surface inclined at a second angle with respect to the axis of the second ringlike core member.
The electromagnetic coupling apparatus further includes an annular exciting coil accommodated in the annular groove of the first ringlike core member; a ringlike armature member arranged in opposed relationship with the first and second inclined end surfaces of the first and second ringlike core members, said armature member having a second outer diameter larger than the first outer diameter; and a cylindrical pressure member provided so as to surround the outer circumferential surface of the first ringlike core member and be movable in a direction of pressing the multiplate brake mechanism as being guided by the first ringlike core member, said pressure member having a first end fixed to an outer circumferential portion of the armature member and a second end engaged with the multiplate brake mechanism. The ringlike armature member has a radially outside end surface inclined at the first angle with respect to the axis of the armature member and a radially inside end surface inclined at the second angle with respect to the axis of the armature member.
Also according to the still further aspect of the present invention, it is possible to obtain a similar effect such that even when the stroke of the cylindrical pressure member changes due to wearing in the multiplate brake mechanism, a change in the air gap between the core member and the armature member can be suppressed. As a result, the control of the attraction force of the core member, or the control of the braking force in the multiplate brake mechanism can be accurately performed without much influence by the aged deterioration.
In accordance with a still further aspect of the present invention, there is provided a drive force distributing apparatus for a four-wheel drive vehicle having a pair of first drive wheels and a pair of second drive wheels, comprising: a fixed housing; a first axle connected to one of said first drive wheels; a second axle connected to the other first drive wheel; an input shaft rotatably mounted in said fixed housing and connected to a drive source; a first planetary gear assembly having a first ring gear operatively connected to said input shaft, a first planetary carrier fixed to said first axle, a first sun gear rotatably mounted on said first axle, and a first planet gear carried by said first planetary carrier so as to mesh with both said first ring gear and said first sun gear; a second planetary gear assembly having a second ring gear operatively connected to said input shaft, a second planetary carrier fixed to said second axle, a second sun gear rotatably mounted on said second axle, and a second planet gear carried by said second planetary carrier so as to mesh with both said second ring gear and said second sun gear; a first multiplate brake mechanism interposed between said fixed housing and said first sun gear; a first electromagnetic coupling apparatus for controllably operating said first multiplate brake mechanism; a second multiplate brake mechanism interposed between said fixed housing and said second sun gear; and a second electromagnetic coupling apparatus for controllably operating said second multiplate brake mechanism; a drive force from said input shaft being distributed among said first axle, said second axle, and said second drive wheels by operating said first electromagnetic coupling apparatus and said second electromagnetic coupling apparatus; said first electromagnetic coupling apparatus comprising: a first ringlike core member fixed in said fixed housing, said first core member having a first annular groove and a first outer diameter; a first annular exciting coil accommodated in said first annular groove of said first core member; a first ringlike armature member arranged in opposed relationship with said first annular groove of said first core member, said first armature member having a second outer diameter larger than said first outer diameter; and a first cylindrical pressure member provided so as to surround the outer circumferential surface of said first core member and be movable in a direction of pressing said first multiplate brake mechanism as being guided by said first core member, said first pressure member having a first end fixed to an outer circumferential portion of said first armature member and a second end engaged with said first multiplate brake mechanism; said first ringlike core member having a first radially outside tapered end surface formed radially outside of said first annular groove and inclined at a first angle with respect to the axis of said first core member, and a first radially inside tapered end surface formed radially inside of said first annular groove and inclined at a second angle with respect to the axis of said first core member; said first ringlike armature member having a second radially outside tapered end surface complementary to said first radially outside tapered end surface, a second radially inside tapered end surface complementary to said first radially inside tapered end surface, and a first intermediate end surface opposed to said first annular exciting coil; said second electromagnetic coupling apparatus comprising: a second ringlike core member fixed in said fixed housing, said second core member having a second annular groove and a third outer diameter; a second annular exciting coil accommodated in said second annular groove of said second core member; a second ringlike armature member arranged in opposed relationship with said second annular groove of said second core member, said second armature member having a fourth outer diameter larger than said third outer diameter; and a second cylindrical pressure member provided so as to surround the outer circumferential surface of said second core member and be movable in a direction of pressing said second multiplate brake mechanism as being guided by said second core member, said second pressure member having a third end fixed to an outer circumferential portion of said second armature member and a fourth end engaged with said second multiplate brake mechanism; said second ringlike core member having a third radially outside tapered end surface formed radially outside of said second annular groove and inclined at a third angle with respect to the axis of said second core member, and a third radially inside tapered end surface formed radially inside of said second annular groove and inclined at a fourth angle with respect to the axis of said second core member; said second ringlike armature member having a fourth radially outside tapered end surface complementary to said third radially outside tapered end surface, a fourth radially inside tapered end surface complementary to said third radially inside tapered end surface, and a second intermediate end surface opposed to said second annular exciting coil.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.